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Seasonal Catch-Rates BLACK MARLIN The world famous concentration of black marlin off northeastern Australia during October-December is obvious in these maps (in fact, the Howard and Ueyanagi maps were a major factor stimulating the development of the Cairns fishery). In comparison, catch-rates in the same area of the northwest Coral Sea are relatively low in September and January. Longline catch rates of black marlin elsewhere in the region are, in general, relatively low. Localised patches of higher-than-average catch-rates occur off the southeast tip of Papua New Guinea in January and February (perhaps fish returning from the spawning grounds?) and around the southeastern islands of the Solomons in May - June. From August to December, catch-rates are generally higher in the New Britain area than in the Solomons further south. Recaptures of fish tagged off eastern Australia and anecdotal information suggest that there may be major concentrations of immature black marlin (<60kg) around the island archipelagoes of southeast Papua New Guinea, particularly during the South East Trade season. The reason these concentrations have not shown up in the longline data may be that they occur mostly in shallow waters (<200m), inaccessible to the longliners. Captures of black marlin recorded by the Moresby Gamefishing Club support more general findings from the Australian Gamefish Tagging Program that, outside Australian waters, the smaller fish (<100kg) tend to be captured during the South East Trades and larger fish during the North West Monsoon. The average weight of black marlin recorded by the Moresby Gamefishing Club is 45kg during the South East Trades and 125kg during the North West Monsoon (a total of 39 fish weighed). The New Guinea Coastal Undercurrent (NGCU) would seem to offer an obvious route for juvenile fish from shelf waters of northeast Australia and for post-spawning fish to move to Papua New Guinea and the Solomon Islands. The data are presently too sparse to test this idea more thoroughly. Further information on the seasonal distribution of fish by sex and size would help to elucidate patterns of movement. The possibility of immature females leaving the region via the NGCU/South East Monsoon Current and returning as more mature fish via the North Equatorial Counter Current/ North West Monsoon Current is of particular interest. The seasonal concentration of fish in the Queensland Trough (and in the Okinawa Trough in the East China Sea) suggests one might look for concentrations of black marlin in other Troughs in the region, but no such concentrations are evident.
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